How to add an Image Reference to for Side/Front/Top
Press N to bring the properties panel, then enable Background images and select the image.

By default using an image as background will enable the image for all of the preset views.

By selecting the Axis you can restrict the image to appear in only one view.



To have more than one image simply add more images and specify an axis for them:



Remember that Background images will only be displayed in:


Camera Perspective view (Numpad 0)
Any of the preset orthographic views which you can access using the keys on the number pad:


Front/Back (Numpad 1 or CtrlNumpad 1)
Right /Left (Numpad 3 or CtrlNumpad 3)
Top/Bottom (Numpad 7 or CtrlNumpad 7)



Background images will not show in perspective view. To switch between perspective and [orthographic][3] view use Numpad 5

Background images are used for reference only and will not be part of the final render.